Sedimentological and geochemical properties of authigenic carbonates in Kyushu, Japan: Implications for the transition from semi-arid to humid climate during the Eocene

The timing of the East Asian monsoon system has attracted attention because of the possible relationship between the tectonic uplift of the Himalaya and Tibetan Plateau and climatic evolution in East Asia. Although the existence of the East Asian monsoon in the late Eocene has recently been suggested, few paleoclimatic records have been reported from the mid-latitude coastal area in East Asia where the climate might have changed dramatically when the East Asian monsoon was established. In this study, we examined the climatic conditions in the early and late Eocene at two localities in the northern part of Kyushu, Japan, focusing on the depositional environments of freshwater carbonate nodules. The lower Eocene Akasaki Formation yields dolomite and calcite nodules in the red mudstone layers, which record low δ13C and variable δ18O. The geochemical and textural features of the nodules suggest a pedogenic origin, which we interpreted as indicating formation on a floodplain with well-drained soils under a semi-arid climate with seasonal precipitation. In contrast, the upper Eocene Yoshinotani Formation yielded nodules composed of siderite with distinctively positive δ13C and stable δ18O values, suggesting that they developed on wetlands covered by poorly drained soils in a temperate climate with stable rainfall throughout the year. These two types of nodule-bearing formations imply that the climate became wet in the middle Eocene. This climate change in Kyushu predated the rapid global cooling at the Eocene-Oligocene transition and may be related to the appearance of the monsoon system in East Asia, which was caused by the uplift of the Himalaya and Tibetan Plateau. •Pedogenic nodules formed under semi-arid climate in the early Eocene in Kyushu.•Siderite nodules formed in wetland soils in the late Eocene in Kyushu.•Climatic changes in Kyushu may be related to the uplift of the Tibetan Plateau.